The striking arrangements of leaves and flowers, otherwise known as phyllotaxis, has fascinated artists, mathematicians and biologists alike for hundreds of years. These arrangements reflect the fact that plants generate new leaves and flowers in a periodic and continuous manner. Previously our lab discovered that underlying this periodicity is a feedback loop between the hormone auxin and its polar, cell to cell transport. Specifically, we found that localized expression of the MONOPTEROS (MP) transcription factor, which activates genes in response to auxin, was able to orient the polarity of PIN1, the auxin efflux carrier, non-cell autonomously.
Although several hypotheses have been proposed to explain how the polarity of PIN1 is coordinated between cells, experimental evidence testing their assumptions is lacking. Furthermore, the downstream targets of MP that mediate polarity changes have not been identified. Here we report the use of genetic mosaics and live-imaging to address these questions. Our results argue against cell polarity models based on auxin flux and enable us to identify a downstream target gene that, like MP, is able to influence PIN1 polarity in a non-cell autonomous fashion.